Numerous plasma-producing devices are known in the state of the art. For example, French patent FR 85/08 836 describes a technique for plasma excitation at electron cyclotron resonance. Electron cyclotron resonance is obtained when the frequency of gyration of an electron in a magnetic field that is static or quasi-static is equal to the frequency of the applied accelerating electric field. Resonance is obtained for a magnetic field B and an excitation frequency f given by the relationship: EQU B=(2.pi.mf)/e
where m and e are respectively the mass and the charge of an electron. By way of example, at a frequency of 2.45 GHz, a magnetic field of 0.0875 Teslas is required to obtain resonance.
For plasma excitation, electron cyclotron resonance is possible only if electrons can be accelerated sufficiently by this process, i.e. if electrons can rotate long enough in phase with the electric field to acquire the threshold energy required for ionizing the gas. To achieve this, it is necessary firstly for the radius of gyration to be small enough, in particular to remain within the region of space in which the conditions for resonance are united, i.e. a region in which the applied electric field and the magnetic field of intensity B are present simultaneously, and secondly for the frequency of gyration to remain large compared with the frequency of elastic collisions between electrons and neutral elements, i.e. atoms and/or molecules. In other words, the best conditions for plasma excitation at electron cyclotron resonance are obtained when the gas pressure is low enough and simultaneously the electric field frequency f is high enough, i.e. also for a high magnetic field intensity B. In practice, in a conventional plasma, conditions favorable to excitation at electron cyclotron resonance are obtained for frequencies f at about or greater than 500 MHz and for gas pressures of about 10.sup.-1 Pascals, typically 10.sup.-3 Pascals to 10 Pascals, depending on the nature of the gas. Nevertheless, microwave frequencies greater than 10 GHz requires very high magnetic field intensities that cannot be obtained with conventional magnetic structures and permanent magnets. At the frequency f=2.45 GHz, the intensity B is 0.0876 Teslas, and it exceeds 0.35 Teslas at the frequency f=10 GHz.
As can be seen more clearly in FIG. 1, the technique described in the above-mentioned French patent requires, the use of permanent magnets 1 each creating at least one surface 2 of constant magnetic field of an intensity that corresponds to electron cyclotron resonance. Electromagnetic power is conveyed to the resonance zone 2 by antennas 3 or plasma exciters, each constituted by a metal wire element. Each exciter 3 overlies permanent magnets 1 which are mounted on the wall of a sealed enclosure 4.
Both the electromagnetic field and the magnetic field of intensity equal to the value that gives resonance are localized and confined essentially in the space situated between an exciter 3 and the portion of the enclosure wall that overlies a magnet. In the presence of a gaseous medium at low pressure, electrons are accelerated in the resonance zone and they wind around magnetic field lines 5 which define a plasma confinement surface. These field lines 5 form festoons connecting the pole of a magnet to the poles of adjacent magnets. Along its path, an electron dissociates and ionizes the molecules and atoms with which it comes into collision. Plasma is then produced along the field lines and subsequently diffuses from the field lines so as to form a cold plasma that is practically free of energetic electrons which remain trapped in the festoons. A major drawback of such a device is that microwave energy propagation and the resonance zone in which microwave energy is absorbed are superposed. It is therefore not possible for microwaves to propagate along the wire applicator without absorption taking place simultaneously. Therefore, both plasma density and microwave electric field intensity decrease progressively along the antenna. As a result, a plasma is obtained which has non-uniform density along the antenna, and which is consequently unsuitable for most industrial applications.
To remedy this drawback, patent FR 91/00 894 proposes placing the antenna 3 in an inter-magnet zone 6 lying between the wall of the enclosure and the magnetic field lines 5 interconnecting two adjacent poles of different polarities. The zone 6 is particularly propitious for microwave propagation since it is practically free of plasma since plasma diffusion is normal to the field lines and is considerably reduced when the intensity of the magnetic field increases. Standing waves of constant amplitude are thus obtained all along the microwave applicator with minima and maxima of microwave power every half-wavelength. However, even if the microwave power along the applicators is thus uniformly distributed on average, the plasma source is actually made uniform because of the existence of electron drift along the applicators due to the gradient and to the curvature of the magnetic field close to the magnetic field applicators. A uniform plasma can thus be produced along the applicator.
A major drawback of that technique is that the zones where the microwave electric field is at a maximum, i.e. between the applicator and the wall of the enclosure, do not coincide with the resonance zones where the intensity of the magnetic field is equal to electron cyclotron resonance. In order to produce plasma excitation, it is necessary either to increase the intensity of the applied microwave electric field, or else to increase the intensity of the magnetic field to extend the resonance zone. In which case, it is necessary to use permanent magnets capable of delivering very high magnetic field intensities that are considerably greater than those required merely for satisfying conditions of resonance.
In addition, all of the techniques described by the above two patents also suffer from the following drawbacks:
a low percentage of working volume for the magnetic field produced by the permanent magnets; PA1 the need to position the microwave applicators very accurately relative to the magnets; PA1 the need to provide the chamber with walls that are very thin given the very rapid decrease in magnetic field intensity as a function of distance from the surface of the magnet; PA1 the need to use magnetic field applicators that are capable of delivering a magnetic field along the microwave applicator which is as uniform as possible so as to avoid impedance breaks that are extremely unfavorable to the propagation of microwaves along the applicator; PA1 the near impossibility of pumping and distributing gases through the plasma excitation structure; and PA1 poor efficiency of excitation, and pulverization of the walls due to ions which diffuse with the fast electrons that create the plasma and are lost against the walls because of electron drift due to the gradient and to the curvature of the magnetic field (magnetron effect). PA1 a source of microwave energy; PA1 at least one first applicator of microwave energy; and PA1 at least one wire-shaped plasma exciter placed at a distance from the first microwave applicator to define between them an absorption zone, electrons being accelerated by the microwave field along determined trajectories.